As for your experience with Command+Tab not working while Parallels is not the foreground application, I'm afraid I can't help because I haven't experienced that myself.
As for when Parallels is the foremost application, and using Coherence mode:
john.a.evans said:
Sometimes it does, other times it doesnt.
Maybe this has to do with where the mouse pointer is?
What I have found is that once you roll the mouse pointer off of a Coherence window and back on to a Mac background such that the mouse cursor turns from the white windows cursor back into the black Mac cursor, Command+Tab will bring up the application switcher as normal. If the mouse focus is on the coherence window, Command+Tab will no longer bring up the application switcher, as the Command (apple) key is remapped to the windows key.
What is strange is that in the situation I described above, when the mouse pointer "turns black" the keyboard input is still captured by the foremost Parallels window, so it is not as if the keyboard and mouse are being released at this point. Also, Command+H still works to hide Parallels no mater what color the mouse pointer is. Clearly I need to play around with Coherence more, as I have only just begun using it and it seems to have some quirks that require getting used to.
Remember, however, that Coherence is a feature that is still 'in beta' in the sense that it only exists in beta releases and not in any general release. Once the next official build of Parallels is released (which they have generously promised will be a 'dot' upgrade and therefore free for all current users) you can be sure that Coherence will be much more mature.